Within moments of arriving at an adult prison — "starred up" from a juvenile facility that couldn't handle him — Eric (Jack O'Connell) demonstrates how to use jail-issue toiletries to make a weapon. But it's not that toothbrush shiv that makes the 19-year-old deadly. It's his ferocious unpredictability, a quality mirrored by this edgy, naturalistic drama.

Starred Up was directed by Glasgow-based David Mackenzie, who's long been drawn to dangerous characters. Lust is usually the engine in his films, whether they're set on a canal boat (Young Adam) or in a mental institution (Asylum). His latest antihero, however, doesn't seem to think about sex. Eric's tale is an Oedipal fable without a Jocasta.

There are several father figures, though. There's Oliver (Rupert Friend), who runs group therapy sessions for the prison's most violent inhabitants. (Or rather, the most violent who don't wear guard uniforms.) Also taking something of a paternal role is Dennis (Peter Ferdinando), the soft-spoken inmate who runs the penitentiary's rackets. He wants "a nice quiet wing," and has underlings who will kill to restore what passes for peace behind bars.

And then there's Nev (Ben Mendelsohn), who's assigned by Dennis to keep Eric in line. On Nev's cell wall is a child's drawing of a family, with the phrase "I love you, Daddy" scrawled on it. It was made by Eric more than a decade ago, when his father was already incarcerated.

Like his son, Nev is volatile and impulsive. Yet he's learned just how far he can push both the guards and Dennis, and tries to convey this understanding to Eric. The kid, however, has survived thus far by striking first, not holding back. And even if he could adopt a new strategy, he rejects any kind of lesson from his now-hated father, who wasn't around to help after Eric's mother died.

Starred Up has three powerful advantages: a realistic setting, fierce performances and the insight brought by scripter Jonathan Asser, who was an Oliver-like counselor in a tough London jail. American viewers may struggle with the accents and slang, but the movie's principal weakness is its plot, which turns unconvincingly tidy (if not exactly upbeat) in the final act.

Staged in an actual Belfast prison, the drama was filmed in sequence and edited during production so that it was almost finished by the end of the shoot. O'Connell, who's stunning as the always-vigilant yet perpetually off-balance Eric, read the script day by day, so he'd be nearly as clueless as his character.

Cinematographer Michael McDonough shot with a hand-held camera, circling the brawling characters as he's egging them on, and conveying a sense of confinement with carefully framed shots. The colors are harsh, but not muted: Many of the walls are yellow, and the other principal hue is that of faces turned red from rage, pain or asphyxiation.

Probably drawn from the repertoire of Asser's former therapy clients, the movie includes some astonishing scenes of violence, including an unprovoked attack and a standoff in which Eric anchors his teeth in a guard's crotch. Even when he's at rest, the young man conveys menace with every darting eye movement.

Eric remains scary to the end, but a predictable alliance and a few convenient developments ultimately blunt the movie's believability. The script's slide toward prison-flick conventions is not a major problem, however, in a movie that depends less on narrative than on bristling physicality.